Gyroscopes (also referred to herein as gyros) have been used to measure rotation rates or changes in angular velocity about an axis of rotation. A basic conventional fiber-optic gyro (FOG) includes a light source, a beam-generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam-generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams experience different pathlengths while propagating around a rotating closed optical path, and the difference in the two pathlengths is proportional to the rotational rate that is normal to the enclosed area.
In a conventional resonator FOG (RFOG), the counter-propagating light beams are typically monochromatic (e.g., in a single frequency) and circulate through multiple turns of the fiber-optic coil and for multiple passes through the coil using a device, such as a fiber coupler, that redirects light that has passed through the coil back into the coil again (i.e., circulates the light). The beam-generating device modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonator coil may be measured. The resonance frequencies for each of the CW and CCW paths through the coil are determined according to a constructive interference condition such that all light-waves having traversed the coil a different number of times interfere constructively at any point in the coil. As a result of this constructive interference, an optical wave having a wavelength λ is referred to as “on resonance” when the round trip resonator pathlength is equal to an integral number of wavelengths. A rotation about the axis of the coil produces a different pathlength for clockwise and counterclockwise propagation, thus producing a shift between the respective resonance frequencies of the resonator. The frequency difference, such as may be measured by tuning the CW beam and CCW beam frequencies to match the resonance frequency shift of the closed optical path due to rotation, indicates the rotation rate.
To lock the external laser beams to the resonances, sinusoidal phase (or equivalently frequency) modulation is applied to the input beams. However, the modulation of the light beam also has imperfections associated with it. There are at least two types of modulator imperfections that can result in rotation-sensing errors. One type is modulator induced intensity modulation. Even though the intended modulation is either cavity length, optical frequency or optical phase, a non-ideal modulator will also generate a modulation of the light intensity which can have a component at the modulation frequency. The unwanted intensity modulation will be detected by the demodulator and interpreted as a signal indicating an off resonance condition. Resonator tracking electronics will then move the laser frequency away from the resonance frequency until the normal resonator intensity signal exactly cancels out the unwanted intensity signal. The deviation away from the resonance frequency results in a rotation sensing error if the unwanted intensity signals are different between the two counter-propagating light waves.
Another modulator imperfection that can result in rotation-sensing errors is modulation distortion. Modulation distortion can occur at the modulator drive electronics or the modulator. An ideal modulation is a sinusoidal modulation at a single frequency. However, distortion can result in the generation of higher harmonics on the modulation. Even harmonic modulation will result in a resonance detection error which can lead to a rotation sensing error, typically called bias error or simply bias.
One effective way to reduce the bias errors is to use common modulation on the CW and CCW beams so that cancellation of the bias in the two directions can be substantial. This is because the same amount of intensity modulation and modulation distortion are generated on both beams, leading to equal amount of errors that can be subtracted out in the signal processing unit. However, common modulation creates a new challenge to distinguish beams from different lasers. The unwanted back-reflected light of one beam into the path and direction of the other beam will cause new errors that could have been avoided if beams in opposite directions are modulated at different frequencies with separate modulators.
Prior art provides a means for employing common modulation while discriminating between the various optical beams. Balanced heterodyne detection has been taught to provide a means for discriminating between the various optical beams. In this system the various optical beams (slaves) are generated from one master laser. Each slave has a unique frequency separation from the master laser. By interfering the resonator output beams with a portion of the master beam using balanced heterodyne detector, beams from each slave will generate a unique beat note. By using synchronous phase sensitive demodulation techniques, each beat note can be separated from other beat notes, therefore, the signal from each beam can be separated from signals from the other beams while employing common modulation.